Multiplex telegraphy



March 4, 1924 11,485,675

' R.V.L.HARTLEY MULT I FLEX TELEGRAPHY Filed Sept. 29, 1919 sired upper limit,

Patented Mar. 3, i924.

enti e stares RALPH! V. L. HARTLEY, OF EAST ORANGE,

TRIO COMPANY, INCORPORATED, 0F

roan.

, mnnrrrnna NEW JERSEY, ASSIGNOR T0 WETERN MEG- NEW YORK, N. Y., A CORPORATION OF NEW TELEGRAPHY.

Application filed September 29, 1919. Serial No. 327,830.

To all whom it may concern:

Be it known that I, RALPH V. L. HARTLEY, a citizen of the United States, residin at East Orange, in the county of Essex, tate of New Jersey, have invented certain new and, useful Improvements in Multi lea Telegraphy, of which the following is a ull, clear, concise, and exact description.

The present invention relates to the control of wave energy for signaling or similar purposes.

More particularly, the invention relates to the transmission of signals by means of high frequency waves with provision for preventing the transmission of frequencies which are not necessary for efficient signaling, but which are undesirable and which are in general produced incident to the contrql of the high frequency wave by the sig na s.

The invention is particularly applicable to a multiplex high frequency signaling system in which it is advantageous to restrict the transmitted frequencies comprised in eachsignal-controlled high frequency wave to the smallest range of frequencies that are necessary for eflicient signal transmimion, thereby eliminating the interference which is caused between neighboring signaling channels by the extreme frequencies, and permitting a greater number of such chan nels to be included in a given available high frequency range.

By low frequency is meant a frequency of the order of the frequencies at which the signaling impulses occur. In the case of a direct current telegraph line, the low frequency would be the frequency at which the dots and dashes are made. By high frequency is meant the frequency of the sustained wave that is controlled in accordance with the signals for the purpose of transmitting. The high frequency may be anyconvenient frequency from a few hundred cycles per second or less to almost any desay several thousand cycles for transmission on lines and even higher frequencies for radio transmission.

It has been found that when a sustained high frequency wave is chopped into. wave trains to represent the low frequency signaling impulses, or is modified in some other way in accordance with low frequency im pulses, the resulting wave appears, for purposes of selection, frequency but as a wave of the original high frequency accompanied by side frequencies, some of which are not necemary f0 eflicient signaling. In the system to be disclosed in the present application, unessential frequencies are eliminated by acting upon the signal-controlled high frequency wave before said wave is impressed upon the outgoing line.

It is well known that if a sine wave of given high frequency has its amplitude varied sinusoidally at a low frequency rate, the resulting wave may be looked upon as the equivalent of three waves, namely a wave of the original high frequency, a wave of the original high frequency minus the low frequency, and a wave of the original high frequency plus the low frequency. The latter two frequencies are called side frequencies. For any one side frequenc of an original high frequency wave it wi l be seen that there is produced a corresponding side frequency to the other side of said original frequency, and differing from said original frequency by-the same amount as the first mentioned side frequency. Also, if thereare a number of low frequencies instead of one, there will be a number of side frequencies which are termed side bands of frequencies, the high frequency wave together with the side bands of frequencies forming a spectrum co-nsisting'of a p urality of waves of different frequencies. The theory relating to this matter may be found in British Patent,- 102,503. A voice-modulated high frequency wave, such as is met with in carrier current telephony, as is Well known, comprisesside bands of frequencies of considerable width since the voice wave is a complex wave comprising'a comparatively .wide range of frequencies. In carrier telephony advantage 1s taken of this fact, and it is customary, for instance, to eliminate from the modulated wave to be transmitted one or the other of the side bands, since only one such band is necessary at the receiver.

The chopping or otherwise controlling of a high frequency wave by telegraphic impulses may be looked upon as a process of modulation in which the amplitude of the high frequency .wave is varied in accordance with a rectangular wave consisting of intervals of no current alternating with not as a wave of single till) we, a

*duces a wave ,the high frequency. 930

periods of direct currentof definite magnitude. A rectangular wave of definite periodicity; as is well known, may be regarded as a sine wave of a fundamental frequency with a number of other sine waves of successively higher frequency superposed upon it. A given sine wave with another sine Wave of three times as high a frequency properly combined with it, for example, approximates a rectangular wave. By properly choosing and combining with the wave sine waves of still higher frequencies, it is possible to approximate a rectangular wave to any desired degree of exactness. The chopping of the high frequency wave protherefore which consists in effect, of the original high frequency and side bands of frequencies lying to either side of In accordance with the invention, one of the two side-bands resulting from the interruption of, or otherwise controlling, a high frequency wave by telegraph. signals is suppressed from transmis sion, the other side-band being transmitted. By thus suppressing one of the side-bands, an economy may be efiected in the range of frequencies used over what would be required if both side-bands were transmitted for the same quality of transmitted signals.

The more nearly rectangular is the telegraphic signal wave, that is, the more abrupt are the start and finish of theimpulses, the broader will be the side bands resulting from the control of the high frequency wave by the signal impulses. In the case of telegraphy, however, these side bands are much more narrow than in the case of a voice modulated wave. This necessitates the use of selective circuits of such sharply defined characteristics as to discriminate between the side bands of each high frequency wave if one of said side bands is to be transmitted while the other is suppressed.

It is often very important to include as large a number of telegraph channels as possible in a given frequency range. For instance, in composite lines which may be used for transmission of ordinary telephone and telegraph currents and for carrier telephone currents, the range of frequencies available for high frequency telegraph transmission is often limited. Also, in any line whether of the composite type or not, it is desirable to include a great number of telegraph channels in the comparatively lower frequency range since the loss by line attenuation is less at these frequencies and the selection between frequencies spaced by a given frequency interval is more easily accomplished in the lower frequency range than if the frequencies are relatively high. A further advantage of eliminatin bands in carrier telegrap y lies in the fact that with a single side band the amplitude of the detected low frequency components is complete in Fig. lconnected to lay 13 pulls up its armature,

one of the side less dependent upon the phases of the received current than when both side bands" are present.

In the drawing, lF'i 1 shows a transmitting circuit for a hig frequency telegraph system for eliminating one of the side bands, and Figs. 2 and 3 show alternative cir'cuit arrangements.

Tn Fig. 1 a multiplex high frequency line 1 is adapted for simultaneous sending and receiving of several high frequency'waves, each controlled by signals. For this purpose the line is provided with a receiving branch 2 and a transmitting branch 3 connected to the line 1 in accordance with the usual two-way arrangement comprising the conjugate coils 1 and 5' and the line balance 6, so that branch 2- is neutral with respect to current impressed on the line 1 from branch 3. Branch 3 may have several transmitting circuits connected to it, two only of which are indicated. Each transmitting circuit comprises a source of high frequency waves, a circuit for controlling the high frequency waves by signal impulses, also preferably an amplifier, and selective circuits, of which will be described presently more in detail. The receiving circuits, which would normally be associated to branch 2, are not illustrated since they may be of any suitable and well known type and form no part of the present invention. The transmitting circuit for one high frequency channel is shown the transmitting branch 3 through the tuned circuit 6. In this transmitting circuit, the oscillator O is shown as of a well known thermionic type comprising the discharge device 8, having its electrodes related to a circuit including coils 9 and condenser 10. By the well known action of this oscillator, a susall tained wave is generated in the circuit in- I clilding elements 9 and 10. The frequency of the wave may be governed by the adjustable condenser 10, and the coil 9 may serve to couple the generator transmittin amplifier 11 through the coil 12. Normal coil 12 is short-circuited through the back contact of relay 13, and the voltage impressed on the amplifier 11 is, therefore, substantially zero. When the key 14: which controls the circuit of relay 13 through battery 15 is depressed however, re-

opening the short circuit at the back contactof the relay and permitting the voltage induced in coil 12 from the oscillator O to be impressed on the input terminals of amplifier 11. This method of controlling the high frequency wave in accordance wlth low frequency impulses is illustrated by way of example only, and any other suitable manner of accomplishing the result may beused. The amplifier 11 is of the well known thermionic type containing a hot filament, a grid and to the circuit of the an anode as shown,

reserve and serves to amplify the wave from generator before the wave is transmitted to the line 1. In the output of the amplifier is a coil 16 coupled to the circuit 6 andshunted by condenser 17. The circuits 16, 17 and 6 together form a resonant transformer having a natural frequency' at which it transmits energy at maxi will, therefore, be transmitted to the line at the maximum efliciency of the resonant 'cir cuits, and the other wave components will be suppressed to an. extent depending upon their remoteness from the resonant freuency and the sharpness of the tuning of t e selective circuits 6, 16 and 17. The resonant circuits are preferably adjusted to transmit one side band and a considerable amount of oscillator frequency and to suppress practically entirely the other side band, or they may be adjusted so as to transmit practically only one side band but to suppress the. oscillator frequency and the other side band. If the carrier is su pressed at the transmitter, it is necessary, 0 course, to supply it at the receiver, as is explained in the British patent above cited.

In Fig. 2 another means of discriminating sharply between the transmitted side band and suppressed side band or suppressed oscillator frequency and side band, is shown. The oscillator and the signal-control circuit are assumed to be the same as "are shown in Fig. 1. Also, the amplifier 11 may be the same as in Fig. 1, but the space current is supplied from source 18 through a choke coil 19 and the variable output component is thereby caused to pass through the large condenser 20. The output circuit of amplifier 11 is connected to the band filter comprising, respectively, the series and the shunt tuning elements 21 and 22 which are so chosen that the filter as a whole transmits the desired narrow side band of frequencies to be impressed on the line but suppresses those frequencies comprising the opposite side band. The shunt tuning elements 22 may be anti-resonant circuits, and the series tuning elements 21 may be simply condensers, or any other of the well known arrangements for making up the band filter may be employed. For detailed information as to the construction and design of such a filter, the United States patent to G. A. Campbell, No. 1,227,113, issued May 22, 1917, may be consulted. The output of the band filter is through the coil 23, coupled with the tuned circuit 6.

In Fig. 3 there is shown a selective sys tem, which is particularly adapted for .the system of the invention. Here the output of the amplifier 11 for the variable wave is through condenser 20 as in the case of the .arrangement of Fig.2. The circuit for selecting one side band of the signalcontrolled wave and suppressing the other side band consists of a high-pass suppression filter 24 in series with a low-pass suppression filter 25. This particular arrangement of filters to, secure a sharply defined band of transmitted frequencies is described and claimed in the co-pending application of Thornton C. Fry, Serial No. 326,987, filed September 27, 1919. lit may be briefly pointed out here, however, that by a high-pass filter is meant one which freely transmits all frequencies higher than a certain frequency known as the cut-ofi frequency of the filter and depending for its value upon the design of the filter, but which suppresses and substantially extinguishes frequencies lower than the cut-ofi frequency. A low-pass filter conversely is one having a cut-ofi' frequency above which all frequencies are suppressed and below which all frequencies are freely transmitted. It is characteristic'of suppression type filters, such as 24: comprising series tuning elements 26 and shunt tuned circuits 27, or such as filter 25 comprising series antiresonant or loop circuits 28 and shunt tuning elements 29, that the cut-oil frequency is very definite and that the attenuation of the filter increases very ra idly for frequencies to one side of the cut-o frequency, while the transmission property of the filter increases very rapidly for frequencies to the other side of the cut-ofi frequency. By combining a highpass with a low-pass filter of this type, each boundary of the transmitted band is sharply defined and it is, therefore, possible to dis-' criminate between frequencies relatively near together. As has been stated, the side bands in carrier telegraphy are relatively narrow and the selecting arrangement of Fig. 3'may, therefore, be used to advantage for selecting between side bands of a telegraph controlled high frequency wave. I

sections of the filters are indicated, but it will be understood that any desired number-- of sections may be employed, and in general, the larger is the number of sections that are employed the more complete will be the suppression of those frequencies which the filter is designed to attenuate.

While the invention has been illustrated and described with reference to a particular circuit arrangement, it is to be understood that the invention is not to be limited either to the circuit as a whole or to the In Figs. 2 and 3 only a comparatively few sultant frequencies frequency specific details, but only by the scope of the appended claims.

What is claimed is:

1 The method of telegraph signaling whlch comprises controlling a high frequency sustained wave in accordance with a telegraphic signal and passing the resulting wave through a selective circuit to suppress resultant wave components having frequencles lying to one side of said high frequency, and to transmit the corresponding wave components to the other side of said high frequency.v

2. The method of telegraphic signaling which comprises controlling a high frequency sustained wave in accordance with a telegraph signal, and passing the resulting wave through a selective circuit which transmits a wave of said high frequency and a restricted band of com onents of the telegraph controlled wave lying to one side only of said high frequency.

3. In a high frequency telegraph system, a generator of sustained waves, a'circuit for controlling said waves in accordance with a telegraphic signal, a line for transmitting the signal-controlled wave, and selective circuits between said circuit and said line for transmitting only a wave of said high frequency and a portion of the wave components resulting from the action of said impulses upon the high frequency waves lying to one side of said high frequency wave.

4. In a multiplex high frequency telegraph system comprising a plurality of high transmitting sets, each for controlling a given high frequency wave in accordance with a telegraph signal to produce upper and lower side bands of said given high frequency wave; means for transmitting a. component of one of said bands having a low attenuation for said component and having a high attenuation for the corresponding component of the other of said hands.

-5. An oscillator, an amplifier, means for intermittently applying said oscillator to said amplifier, and selective circuits associated with said amplifier for transmitting only the oscillator frequency and the relying to one side of the oscillator frequency.

6. In a multiplex high frequency telegraph system comprising transmitting circuits, eachlfor controlling a given high frequency wave in accordance with telegraph signals impulses, the method which comprises selecting'from the signal-controlled wave for signal transmission, only those 'quency anchto 'lator frequency.

ea ers wave which lie within a range whosecen- W tre lies to one side of the frequency spectrum of the signal-controlled wave.

7. A pair of conductors, an oscillator associated therewith, means for intermittently applying waves derived from said source to said conductors,- and a band filter inserted between said means and said conductors and adjusted to transmit frequency components arising from the intermittent control of said waves and having frequencies lying to one side of said oscillator fresuppress correspondin wave the other side of sai oscilfrequencies to 8. The method of telegraphy by sus tained waves, which comprises throwing trains of waves on a transmitting circuit and suppressing transmission of said waves, in succession and in an abrupt manner, and filtering the transmitted wave to restrict the frequency components that are transmitted to the components which are essential upon detection to the production of substantially square-topped impulses.

9. The method of telegraphy by sustained waves. which comprises abruptly establishing and suppressing the How of waves into a transmitting circuit to indicate telegraphic signal elements, and in filtering the transmitted wave to suppress thetransmission at all times of substantially all frequency components lying to one side of the wave frequency, and to restrict the transmitted frequencies on the other side of the wave frequency to those components which difi'er in frequency from the wave frequency by the frequency of recurrence of the signal elements and relatively few harmonics of said frequency of recurrence.

10; In a multiplex carrier telegraph sys- 11cc tem, a line, v a plurality of sources of sustained waves of different frequencies, means associated with each of said sources for abruptl establishing and suppressing the flow 0 waves from the respectlve wave not source to said line in accordance with telegraph signals, and selective circuits interposed between said sources and said line, each selective of the-respective wave frequency and a band of frequencies lying to no oneside of the wave frequency but ofiering high attenuation to the corresponding frequencies lying to the other side of the respective wave frequency.

In witness whereof, hereunto subscribe my name this 27th day of September, A

RALPH v. HARTLEY. 

